(1) Field of Invention
The present invention relates to a robotic control system and, more particularly, to a system for controlling motion and constraint forces in a robotic system without the need for force sensing.
(2) Description of Related Art
Robotic control in complex environments increasingly demands high degree-of-freedom robots that can perform task-level motion commands in the presence of multiple interactions with the physical environment. These interactions with the environment may be intermittent (e.g., robot legs making contact with the floor, robot arms interacting with objects or other robots, etc.) or persistent internal constraints associated with the structure of the robot (e.g., parallel kinematic structures involving loop closures in the robot's kinematic chains).
Traditional joint space control is ill suited to address constraints since the constraints restrict robot motion to a subset of joint space. Joint space control assumes the entire joint space is accessible, consequently, a given joint space command will likely violate the system constraints.
Several research efforts have dealt with constraints in robotic control. for example, Huston, R. L., Liu, C. Q., and Li, F., in “Equivalent control of constrained multibody systems,” Multibody System Dynamics, 10(3), 313-321 (2003), addressed constraints in the context of equivalent motion control; that is, determining generalized force systems that, although different, will have the same dynamic effect. The approach of Huston, Liu, and Li, while providing insight into how certain generalized forces can be nullified by system constraints, is rooted in a joint space analysis and is not well suited to a unified constrained task-level control framework.
Coordinated multi-arm movements was described by Khatib, O., Yokoi, K., Chang, K., Ruspini, D., Holmberg, R., Casal, A., and Baader, A. (1996), in “Force strategies for cooperative tasks in multiple mobile manipulation systems”, Robotics Research (pp. 333-342) (1996), Springer London. Khatib et al. demonstrated the augmented object and virtual linkage model, while Chang, K. S., Holmberg, R., and Khatib, O., in “The augmented object model: Cooperative manipulation and parallel mechanism dynamics,” IRobotics and Automation, 2000, Proceedings, ICRA'00. IEEE International Conference (Vol. 1, pp. 470-475), IEEE, demonstrated a process for manipulating objects with multiple arms and controlling internal forces in the system. The work of Chang et al. is based on the operational space approach, a task-level control formalism; however it does not offer the flexibility of dealing with general holonomic constraints, or passive joints (many closed chain robotic structures consist of unactuated, or passive joints). The whole body control framework of Khatib, O., Sentis, L., Park, J., and Warren, J., in “Whole-body dynamic behavior and control of human-like robots,” International Journal of Humanoid Robotics, 1(01), 29-43 (2004) and Sentis, L., Park, J., and Khatib, O. in “Modeling and control of multi-contact centers of pressure and internal forces in humanoid robots,” Intelligent Robots and Systems, October 2009, IROS 2009, IEE/RSJ International Conference (pp. 453-460) IEEE, is also based on the operational space approach, but also suffers in that their work does not offer the desired flexibility.
As evident above, the state of the art is directed to using operational space or joint space control with force servoing when dealing with constraints imposed by environmental interactions; or to using ad hoc inverse kinematic solutions and joint servoing when dealing with internal mechanism constraints such as in a parallel mechanism. While operable to some extent, the prior art is limited in that it does not provide for constrained task-level motion and force control to provide the desired flexibility.
Thus, a continuing need exists for a system for controlling motion and constraint forces in a robotic system that results in a unified and flexible technique for motion and force control.